Seeing how there is an even cut at the cable, kinda, sorta, suggests to me that the clamp may be too tight. Hard to say for sure though. I installed some of these "Alligator, 31 strand Superior Shine" for my 6600 shifters which usually fray at the shifter. They use more but thinner strands than Shimano or Jagwire and are compatible with both housings and cheaper too. I'm hoping that the thinner strands make them less prone to break from the constant bending at the shifter drum and maybe better for the OP's derailleur situation. I only have about 2K miles on them so far and the Shimano and Jagwire's best cables would usually last around 4-6K miles so can't yet say if they're actually better but worth a try in your case.
ALLIGATOR 31-STRAND INNER CABLE (alligatorcables.com) I found the best prices were on Ebay.